I'm not sure I agree with having a single designer in a team. But they are hard to get, and it is much better to have one than none in an engineering world :)
Plus, also not sure I agree with having the design specifically done before implementing the feature. If clear interfaces are specified I believe this work can be paralellized. There seems to be no way for the rest of the team to contribute to this part of the customer value following the approach described in this article.
One thing that was also not so clear to me is whether this design includes prototyping of the UI and, therefore the UX.
Still, it is a very interesting approach from where to take cool ideas!